In a conventional laser welding practice, a tapered nozzle (hereinafter referred to as a long nozzle) covers a region from the lenses to the close vicinity of the focal point, to protect the laser beam from a disturbance such as smoke (fume or plasma plume), sputters (scattered molten powder) or the like (hereinafter referred to as floating matters) until the laser beam reaches the focal point (the weld of the work) or to prevent floating matters from sticking to the condenser lens in the upper part or the protective glass underneath it. It is also known that shield gas, indispensable for welding, is jetted from a guiding channel disposed in the long nozzle in combination with it (see Patent Document 1).
Furthermore, while it is necessary in laser welding to position the focal point of the laser beam on the surface of the work (or somewhat within the work) with high precision, it is difficult to keep the focal distance constant while moving the beam in the welding direction, especially so where the weld is not flat. To address this problem, it is also known to provide a roller in a position orthogonal to the proceeding direction of welding relative to the focal part of the laser beam and forcibly keep the focal distance with the roller following along the surface of the work (see Patent Documents 1, 2 and 3).
Also, it is an extensively adopted practice to arrange the guiding channel and jetting outlet (hereinafter referred to as a side nozzle) of shield gas so as to extend very closer to the focal point independently of the main nozzle with the aim of preventing shield gas from flowing in too large a volume and from diffusing.
Since this method inevitably entails an arrangement in which the length of the main nozzle is kept short to avoid interference with the side nozzle and the side nozzle is positioned between the focal point and the short main nozzle, the shielding effect of the long nozzle against floating matters cannot be expected, making the disturbance of the laser beam and the contamination of the protective glass unavoidable.
Known techniques to address this problem include providing another air jetting outlet between the short nozzle and the shield gas jetting outlet and jetting out a substantially horizontal air curtain from there to prevent floating matters from shifting upward (being blown up) (see Patent Documents 4 and 5).
Still other known techniques include the addition of a large duct, slit passage of air (lamination of the air flow) and focusing of inert gas to the focal point (downward orientation) for strengthening the air curtain effect (see Patent Document 6).
Patent Document 1: JP-A-3-110094
Patent Document 2: JP-B2-3312896
Patent Document 3: JP-B2-3482452
Patent Document 4: JP-A-6-122089
Patent Document 5: JP-A-2000-263276
Patent Document 6: JP-A-6-79489